An oil seal for a variable valve timing mechanism (hereinafter also referred to simply as oil seal) is required to have high dimensional accuracy with a view to enhancing sealing property. Therefore, the oil seal is sometimes formed of a sintered metal allowing for forming with high accuracy. In this case, an iron-based sintered metal is often used from the view point of material cost. In general, the iron-based sintered metal is formed by the following procedure: raw material powder obtained by mixing iron powder with trace amounts of graphite powder and copper powder is subjected to compression molding, to form a green compact; and the green compact is sintered at high temperature (1,100° C. or more). Through such procedure, carbon in graphite is diffused in an iron structure to form a pearlite phase, and copper is dissolved as a solid solution in the iron structure. Thus, the sintered compact to be obtained has high strength.
In the sintering of a green compact at high temperature as described above, demanded dimensional accuracy may not be obtained unless the green compact is heated uniformly, because its shrinkage amount varies with location. Therefore, the sintering needs to be performed in a state in which the green compacts are aligned so that their directions and postures are uniformized. However, the green compacts before the sintering have low strength, and hence have a risk of being damaged when being grabbed by a robot hand or the like in the alignment of the green compacts. For example, in Patent Literature 1, green compacts are prevented from being damaged by subjecting the green compacts in a non-aligned state to provisional sintering at relatively low temperature (approximately from 750 to 900° C.), to increase their strength to some extent, followed by sintering the provisionally sintered compacts in an aligned state at high temperature.